An independent Scotland would miss out on billions of pounds in overseas trade deals being brokered by the UK government, William Hague has said.

The foreign secretary said the UK government was stepping up its efforts to win new export markets and sign major trade deals through its 267 embassies and diplomatic posts around the world, backed up with £1.5bn in new export aid for British firms.

Speaking to an audience of businesspeople and academics in Edinburgh, Hague said a vote for independence would exclude Scotland from sharing in these deals, damaging major industries such as Scotch whisky and green technologies.

The Foreign Office had made overseas trade one of its top three priorities, recruiting 300 more diplomats to promote export deals in high growth markets in Asia, South America and Africa, opening up a string of new trade and diplomatic offices, he said.

David Cameron, the prime minister, had directly persuaded the Chinese government to protect Scotch whisky imports while British diplomats were focusing heavily on bringing down 600 other trade barriers against whisky worldwide, Hague added.

"I believe that those who say Scotland would be better going it alone in foreign policy are deeply misguided," he said.

"They claim that the UK's foreign policy and overseas representation is focused too much on status and power, and not enough on trade and investment.

"They argue that our extensive overseas network is merely a legacy of empire, rather than something that serves the interest of all the UK's citizens; and they say that Scotland's core interests would be better served as a separate state.

"They are wrong on each and every count."

Hague's warnings are an effort to counter Alex Salmond's success as Scotland's first minister in building up Scotland's own trading partnerships and trade deals, independent of the Foreign Office and other Whitehall departments.

Salmond has led delegations to major markets such as China, the United States, Germany and India as first minister, using Scotland's own network of trade offices in 18 countries – many of which are based in British embassies.

Scotland is now the UK's second largest recipient for inward investment deals after London and the south-east. A study by Ernst and Young earlier this month said the number of foreign investments in Scotland had hit its highest level in 15 years, taking Scotland's UK share to 10%. The number of jobs created, however, had fallen by 18%.

Hague said that success was in part down to the UK's huge global reach and influence, which would end with independence. More than 500 Scottish firms had used the UK Trade & Investment (UKTI) export agency, which has over 1,000 staff in 100 countries, last year.

"This means Scotland enjoys the flexibility of a smaller nation targeting its resources in specific areas, while also having the strength and certainty of the worldwide reach provided by UKTI," Hague said.

By contrast, it would take an independent Scotland many years and tens of millions of pounds to build up a comparable trade and diplomatic network.

"As part of the United Kingdom Scotland derives - and will continue to derive - many benefits from being part of this global diplomatic network, instead of having to rely on inevitably fewer, smaller embassies which would take time and resources to establish," Hague said.